Smoke or steam coming from grass

Ok I think i know the cause of this but want your opinions. I started raking my yard ( dont have to many leaves). Racked about 1/4 of my yard then all the sudden after one stroke I notices steam or smoke coming from the grass. Not the leaves. Is this just decomp from the grass dying from being covered up. I've helped my friend shovel grass out of his truck bed and seeing all the steam and feeling the heat off the grass but this is a little different. The grass was not warm, didn't look like it was dying. It stayed steaming or smoking for about 3 minutes. One thing I did do was I layed down some winterizer furtilizer about a month ago not sure if that did anything. I know if you lay to much in one spot I could burn the grass but that was a month ago and the leaves just started falling 2 weeks ago. I'm not really worried about this just want some insite. I tried googleing but im sure what kind of results I was coming up with when you try seaching Smoking grass. LOL